The present invention relates to microencapsulated compositions of agricultural chemicals, particularly herbicides. This invention also relates to microcapsules that provide a controlled rate of release of their contents.
Controlled release for biologically active materials has been a topic of intense interest for the agricultural industry. Controlled release delivery systems offer the promise of reductions in pesticide usage and in volatility losses. Pesticide leaching into ground water, a serious problem for all-at-once methods of delivery typically used with emulsifiable and suspension concentrates, could be significantly reduced by a delivery system that provided controlled release. Product toxicity could be improved, and better crop safety achieved. These advantages have led to the development of a number of formulations involving microcapsules and microspheres.
A number of microencapsulation techniques have been developed, and a wide variety of them are used extensively in the graphic arts and pharmaceutical industries. In the agricultural field, however, most commercial techniques are limited to polyurea shell walls formed by interfacial polymerization. Aromatic isocyanates are used exclusively with either a polyamine crosslinker (Beestman, U.S. Pat. No. 4,280,833) or another aromatic isocyanate that is hydrolyzed in-situ to produce the amine (Scher, U.S. Pat. No. 4,643,764). These processes are simple and moderately successful. However these rigid, microporous capsules have not fully realized the promise of controlled release.
The release mechanisms of these polyurea microcapsules are poorly defined. The escape of core material from the capsule has been described as either diffusion through the microporous shell wall or rupture induced by environmental stress. The only practical means of adjusting the release rate from these microcapsules is by modifying the wall thickness or particle size.
Reducing the wall thickness to increase the release rate has definite limitations. The thin walls produced are sensitive to premature mechanical rupture during handling or in the field, resulting in immediate release. Poor package stability can also arise when the core material (i.e., the material inside the shell wall) is in direct contact with the external vehicle through wall defects. Some core materials may crystallize outside the capsule causing problems in spray applications. The product becomes little more than an emulsion stabilized against coalescence. When delivered to the field, the release is so fast that little is gained over traditional emulsion concentrate formulations.
If the wall thickness is increased, the bioefficacy quickly drops to a marginal performance level. There is also a practical limit to the wall thickness in interfacial polymerization. As the polymer precipitates, the reaction becomes diffusion controlled. The reaction rate can drop to such an extent that non-constructive side reactions can predominate. Hydrolysis of the isocyanate by residual moisture in the core is one of the more common side reactions. Since this reaction is not interfacial, there is no assurance that this polymerization contributes to wall formation.
Adjusting the release by changing the particle size suffers from most of the problems associated with changing wall thickness. In one sense, it is simply an indirect means of adjusting wall thickness. Additionally, interfacial polymerization techniques are ideally suited for production of capsules in the 2 to 12 micron range. The release rate does not vary significantly between these two extremes. The limited difference in release rate is further muted by the averaging effects of broadening size distributions that inevitably occur as the size is increased.
These prior art microencapsulation procedures are thus adequate for producing very fast release rates or very slow release rates. The practitioner of this art has great difficulty optimizing the release rates to obtain maximum bioefficacy for a given active ingredient (e.g., a herbicide). Various formulation solutions have been attempted to address this limitation. Two package or single package blends of microcapsules and dispersions or emulsions of free agricultural actives have been proposed (Scher, U.S. Pat. Nos. 5,223,477 and 5,049,182). It is one of the purposes of this invention to provide a microcapsule whose release mechanism and rate are reliable and readily adjustable.